1. Field of the Invention
The present invention concerns generally progressive varifocal lenses, which during the last years have mainly replaced so called bi-focal lenses having a clearly zoned near vision area and a area which otherwise is dedicated for the far vision. Especially, the invention concerns so called individual varifocal lenses, but is not limited thereto.
2. Brief Description of the Related Art
Progressive varifocal lenses are known for several years and comprise in usual manner an area dedicated to the far vision in the above section of the lens, and an area dedicated for the near vision, for example for reading, in the lower section of the lens. Between these two areas, there is the so-called progression zone, in which the mean sphere varies. Both, when designing as well as when describing such lenses, reference is usually made to a far vision reference point, situated in the far vision area, as well as to a near vision reference point or control point, wherein at the said points the respective optical effect in diopters for correcting the ametropia of the wearer is dominant. The two reference points or points of reference are positioned on a so-called main progression meridian, along which the increase of addition takes place. The principal meridian of progression is usually defined based on surface properties as a line proceeding approximately between the 0.5 iso-astigmatism lines of a non-prescription progressive surface. Sporadically, it is also called umbilical line or line of main view, and therefore practically usually coincides with the respective penetration points of the view of a wearer, when he moves his eyes when changing from near vision to far vision and vice versa. At least this is intended when designing varifocal lenses.
In case of the varifocal lenses which are presently available on the market, either in a first step, a surface design is developed, and based on specific surface properties a whole lens family having respective matches of bases and addition values is produced. The optician then chooses the most suitable lens for the wearer, depending on the prescription, and if necessary, carries out a respective adaptation of the lens for the specific needs of the wearer.
As an alternative to the before mentioned varifocal lens families, increasingly frequently so-called individual varifocal lenses are used, which means as much as that for each respective one wearer of lenses, according to physiological data, a varifocal lens is designed and manufactured.
In both cases, it is possible to use one of the surfaces for the provision of the progressive or varifocal surface, and the other surface, for the provision of the spherical and/or torical correction, wherein it alternatively is also possible to provide both corrections at one and the same side, or to distribute the progression to both surfaces, or to combine a progressive surface with a regressive one.
Irrespective the fact, whether the design and manufacturing of a complete lens family or an individual varifocal lens is concerned, extremely complex mathematical calculations, especially calculations of surfaces, are necessary. Details concerning this context may especially be found in the work of Heinz Diepes and Ralf Blendowske “Optik und Technik der Brille”, published in 2002 by the Optische, Fachveröffentlichung GmbH, Heidelberg. There, besides others, is described the common practice for the design and structuring' of varifocal lenses, whereupon the x and y coordinates of a first point are determined in the closed interval between the far vision reference point of the far, vision area and the so-called center point or control point of the lens, whereafter/whereupon the x and y coordinates of a second point are determined as the near vision reference points of the near vision area. As soon as these two points have been defined, one tries to determine a mathematical function to determine a preferably linearly ascending means sphere value, wherein the transition to the respective far vision area(s) and near vision area(s), in which the surface related refractive power or mean sphere does not change anymore, should be effected preferably in a most harmonical manner. Stated differently, one tries to determine a mathematical function which comprises the respective end points as local extremas, and preferably only possesses one single point of inflection between the two points. This approach usually, and as described in the above named work, is approximated by a polynom of third order, which is determined by defining boundary conditions.
Alternatively to the mathematical description, but with adequate result, so-called spline functions may be used as well. In each case, every variation of the lens, i.e. when designing a complete family of varifocal lenses or when providing an individualized varifocal lens requires a considerable demand of calculation to be able to calculate the complete free surfaces, in order to be able to provide, as is common, a preferably homogeneous distribution of the surface's astigmatism or cylinder and of respective vision fields in the vicinity of the” intermediate area and in the far vision area, after setting respective weights or evaluations.
In order to reduce the above named amount of calculation, in Patent EP-B1 0969309 was further proposed to base the proceeding of the principal meridian of progression on an approximation formula, when designing varifocal lenses. According to the disclosure of this patent pamphlet, for the offset X0 of the main line projected into the x/y plane is defined: x0(y)=b″+a″−a″:(1+ec(y+d)), wherein for the coefficients is: 2≦a″≦4.3 mm; 0.03≦b″≦0 mm; −0.4≦c≦−0.3 mm−1; 4.5≦d≦5.1 mm.